The present invention relates to a transmitting power control system and more particularly to a transmitting power control system used for providing a constant power to a satellite during satellite communication.
Rain, occurring on a transmission path between a communication satellite and a ground station, may cause attenuation of transmitted power, thus stopping satellite operation. To avoid such a situation, it is necessary to provide a transmitting power control system which ensures that transmission power is constantly applied to the satellite.
FIG. 1 shows a conventional transmitting power control system comprising a first measurement unit 1, a second measurement unit 2, a first mode operation unit 3, a measurement executable judging unit 4, a second mode operation unit 5, and a switching selector 6. As shown in FIG. 2A, first measurement unit 4 measures degradation amount of a returning signal quality L.sub.T using an originating station returning wave A which runs over a communication path from a ground station X to a communication satellite Y, namely, an up link in which a signal transmission is upward, and a communication path from a communication satellite Y to a ground station X, namely, a down link in which a signal transmission is downward. As shown in FIG. 2B, second measurement unit 2 measures a rain attenuation amount L.sub.B of a down link from a communication satellite Y to a ground station X by using a beacon wave B. First mode arithmetic operation unit 3 arithmetically obtains an actual up-link rain attenuation amount Lu based on a degradation amount of a returning signal quality L.sub.T and a down link rain attenuation amount Ld. Measurement executable judging unit 4 determines whether the measurement is possible or not, based on an originating station returning wave. Second mode arithmetic operation 5 arithmetically assumes and obtains an up-link rain attenuation amount Lu.sub.A based on a down link rain attenuation amount Ld when measurement executable determining judging unit 4 determines that the measurement is impossible. Switching unit (selector) 6 produces a transmitting power control signal by switching the output from the first mode operation unit 3 to the output of the second mode operation unit 5 when the measurement executable judging unit 4 determines that the measurement is impossible.
In such a conventional system, a total degradation amount of a returning signal quality L.sub.T for an up-link and a down-link is measured by providing an originating station returning wave A to first measurement unit 1 and a rain wave attenuation amount Ld for a down link is measured by providing a beacon wave B to second measurement unit 2.
A degradation amount of a returning signal quality L.sub.T measured by first measurement unit 1 and a rain attenuation amount Ld for the down link measured by second measurement unit 2 is provided to first mode arithmetic operating unit 3 to perform an arithmetic operation on a rain attenuation amount Lu.sub.A for an up link.
In this case, an up-link rain attenuation amount Lu.sub.A can be obtained as an actual value in accordance with the following theoretical equation designating that the up-link rain attenuation amount Lu.sub.A is a function of L.sub.T and Ld.
Lu.sub.A =f (L.sub.t, Ld) . . . (1)
A rain attenuation amount Ld for a down link measured by the second measurement unit 2 is provided to the second mode, arithmetic operation unit 5 to arithmetically obtain a rain attenuation amount Lu.sub.B. In this case, a rain attenuation amount Lu.sub.B for an up link is presumed according to the following equation.
Lu.sub.B (dB)=K.times.Ld (dB) . . . (2)
where, K is a constant.
In this case, a relation between a rain attenuation amounts Lu and Ld is actually distributed as shown in the experimentation graph shown in FIG. 3. The accuracy of equation (1) is better than that of equation (2).
On the other hand, rain attenuation amount Lu.sub.A for the up-link obtained in accordance with FIG. 2A is more limited in respect of measurable scope than rain attenuation amount Lu.sub.B for the up-link presumed in accordance with FIG. 2B. Actually, if a bit error rate is used to detect degradation amount of the originating station returning wave quality, the scope in which an appropriate bit error is generated within an actual measurable period is limited.
Accordingly, measurement executable judgement unit 4 determines whether or not the originating station returning wave A can be used for the measurement. If it determines the measurement is impossible, the up-link rain attenuation amount Lu.sub.B obtained from the second mode arithmetic operation unit 5 is outputted from selector 6. If it determines that the measurement is possible, the up-link rain attenuation amount Lu.sub.A obtained by the first mode arithmetic operating unit 3 is outputted from selector 6.
The output from selector 6 is provided to an amplifier D for performing a power amplification of a transmitting signal from a transmitter (not shown). The transmitting power control signal from the transmitting power control apparatus C, shown in FIG. 4, is further transmitted from the antenna X through an orthogonal mode transducer.
However, in such a conventional transmitting power control system, two mode operation units for obtaining an up-link rain attenuation amount performs operations independently from each other. The outputs of these two mode operating units are selected by selector 6 depending on whether or not the originating unit returning wave can be used for the measurement. Therefore, there is a problem that the transmitting control power becomes discontinuous upon switching the power of selector 6.